Commentary Aristotle Physics
But, if it be said that the whole moves itself by reason of the whole, then it will be only per accidens that some parts move themselves. But what is per accidens is not necessary. Therefore, in the mover that first moves itself, it is most important to presume that the parts are not moved by themselves. Therefore, one part of the first mover that moves itself will cause motion, since it is immobile, and the other will be moved. For those are the only two ways in which it is possible that a part that causes motion could be moved, namely, either because that part would be moved by another part that it moves or because that part would move itself.
Hence, it should be noticed that Aristotle, in excluding these two ways, intends to conclude that, in a thing that moves itself, the part that causes motion is immobile, but not that what moves itself is divided into two parts, one of which causes motion and the other of which is moved; for this had been sufficiently concluded when he first proved that the whole does not move itself as a whole.
Accordingly, it is clear that it was not necessary that Aristotle introduce a division of five members, as some claimed:
one member of which is that the whole moves the whole;
the second that the whole moves a part;
the third that a part moves the whole;
the fourth that two parts mutually move one another;
the fifth that one part is a mover and the other moved.
For if the whole does not move the whole, it follows for the same reason that the whole does not move the part, nor the part the whole, for in either case, it would follow that a moved part would be moving itself. Hence, the fact that the whole does not move the whole suffices for concluding that one part is a mover and the other is moved. But, in order to conclude that the part that causes motion is not moved, he proves two other things: that the part causing motion is not moved by a moved part and that it is not moved by itself.
1061. And, to prove this last point, he presents a second argument, at “further, if the whole moves itself” (258a3). If it be granted that the motion-causing part of a self-mover moves itself as a whole, it follows, through what was proved above, that a part of that part causes motion and the other part is moved.
For it has been already proved above that a whole does not move itself in any other way than by one of its parts causing motion and the other being moved. So, let AB be the motion-causing part of a thing that moves itself; then, by the previous argument, it follows that one part of it is a mover, namely A, and the other part, namely B, is moved.
Therefore, if AB as a whole moves itself as a whole, as you say, it follows that the same thing would be moved by two movers—by the whole AB and by the part A—which is impossible. It remains, therefore, that the motion-causing part of a thing that moves itself is entirely immobile2184.
Lecture 11. Relation of the parts of a self-mover to each other and to the whole
And, since what imparts motion may be either a thing that is moved by another or an unmoved thing, and that which is moved may be either a thing that imparts motion to something else or not, that which moves itself must be composed of something that is unmoved but imparts motion and also of something that is moved but does not impart motion of necessity, but may or may not do so.
Thus, let A be something that imparts motion but is unmoved, B something that is moved by A and moves C, C something that is moved by B but moves nothing (granted that we eventually arrive at C, we may take it that there is only one intermediate term, though there may be more). Then the whole ABC moves itself. But, if I take away C, AB will move itself, A imparting motion and B being moved, but C will not move itself or in fact be moved at all. Nor again will BC move itself apart from A, for B imparts motion only through being moved by something else, not through being moved by any part of itself. So only AB moves itself. That which moves itself, therefore, must comprise something that imparts motion but is unmoved and something that is moved but moves nothing of necessity.
And each of these two things, or at any rate one of them, must be in contact with the other.
If, then, that which imparts motion is a continuous substance—that which is moved must of course be so—it is clear that it is not through some part of the whole being of such a nature as to be capable of moving itself that the whole moves itself, it moves itself as a whole, both being moved and imparting motion, through containing a part that imparts motion and a part that is moved.
It does not impart motion as a whole, nor is it moved as a whole; it is A alone that imparts motion and B alone that is moved. It is not true, further, that C is moved by A, which is impossible.
Here a difficulty arises. If something is taken away from A (supposing that that which imparts motion but is unmoved is a continuous substance), or from B the part that is moved, will the remainder of A continue to impart motion or the remainder of B continue to be moved?
If so, it will not be AB primarily that is moved by itself, since, when something is taken away from AB, the remainder of AB will still continue to move itself.
Perhaps we may state the case thus: there is nothing to prevent each of the two parts, or at any rate one of them—that which is moved—being divisible though actually undivided, such that, if it is divided, it will not continue in the possession of the same capacity.
And so there is nothing to prevent self-motion residing primarily in things that are potentially divisible.
From what has been said, then, it is evident that that which primarily imparts motion is unmoved.
For whether the series is closed at once by that which is in motion but moved by something else deriving its motion directly from the first unmoved, or whether the motion is derived from what is in motion but moves itself and stops its own motion, on both suppositions, we have the result that, in all cases of things being in motion, that which primarily imparts motion is unmoved.
1062. After showing that a thing that moves itself is divided into two parts, one of which causes motion and is not moved and the other of which is moved, the Philosopher now shows how such parts are mutually related.
About this, he does three things:
first, he proposes what he intends;
second, he shows his proposition, at thus, let A (258a9; [1063]);
third, he reaches the conclusion chiefly intended by all the foregoing, at from what has been (258b4; [1068]).
He says first (258a5) that, since a mover is divided into two elements, one of which is also moved by something else and the other of which is immobile, and again, since a mobile is divided into two, there being a mobile that also causes motion and another that does not move anything, one must say that what moves itself is composed of two parts, one of which is such a mover as to be immobile, and the other of which is so moved as not to move anything else.
And when he says that the latter does not move anything of necessity, it can mean two things.
If it is understood as though the moved part of a self-mover does not move anything that is part of the self-mover, the word “necessity” should be interpreted in an affirmative sense, referring to his saying it does not impart motion.
For he at once proves that it is impossible for a thing that moves itself to have a third part that is moved by the moved part.
But if the words are interpreted as meaning that the moved part does not move anything extrinsic, then the phrase of necessity must be given a negative meaning; for it is not necessary in a thing that moves itself that its moved part move something extrinsic, but neither is it impossible.
1063. How this happens he shows at “thus, let A” (258a9).
About this, he does two things:
first, he explains his proposition;
second, he solves a doubt, at here a difficulty (258a27; [1066]).
About the first, he does two things:
first, he shows how the parts of a thing that moves itself are related;
second, he shows how, with respect to them, a whole is said to move itself, at if, then, that which (258a21; [1065]).